[Purdue-pm] tweak to program

[Mark Senn]

The program I sent earlier had
    (/^(-?\d+)$/)          and  $begin = $1, $end = $1;
    (/^(-?\d+)-(-?\d+)$/)  and  $begin = $1, $end = $2;
    push @r, $begin..$end;
I changed that to
    /^(-?\d+)(?:-(-?\d+))?$/;
    $begin = $1;
    $end = $2 // $1;
    push @r, $begin..$end;
because I think that expresses it better, even though the regular
expression is more complex.  (I usually don't use the "group but don't
capture"
    (?: ... )
because I don't like that syntax and instead capture everything even if
it is not used later.)

The $2 // $1 means use the value of $2 if it is set, otherwise use the
value of $1.

This code is hard to read if one hasn't seen this technique before but
what I'm trying to express maps nicely to this code.

-mark